"It persists. It doesn't get better, and it involves several things," he explains. "The person is sad, feeling irritable, they really don't have any interests. Pleasurable activities are no longer pleasurable. Their sleep is disturbed frequently, they end up sleeping a lot more, and they never really feel like they are sleeping enough or resting enough."

Doctors believe SAD can be triggered by a lack of vitamin D, which comes from the sun. It can be treated with anti-depressants and vitamin D supplements. A light therapy box can also mimic sunlight during the shorter days of winter.